Many audio recordings nowadays are available in stereo or in so-called 5.1-surround format. For playback of these recordings, two loudspeakers in the case of stereo, or six loudspeakers in the case of a 5.1-surround are necessary as well as a certain standard speaker set-up.
However, in many practical cases, the number of loudspeakers or the set-up does not meet the requirements to achieve a high quality audio playback. For that reason, audio redistribution systems have been developed. Such an audio redistribution system has a number of N input channels and a number of M output channels. Thus, three situations are possible:
In a first situation, M is greater than N. This means that more loudspeakers are used for playback than there are stored audio channels.
In a second situation, M is equal to N. In this case, equal numbers of input and output channels are present. However, the speaker set-up for playing back output is not in conformity to the data provided as an input, which requires redistribution.
According to a third scenario, M is smaller than N. In this case, more audio channels are available than playback channels.
An example of the first situation is the conversion from stereo to 5.1-surround. Known systems of this type are Dolby Pro Logic™ (see Gundry, Kenneth “A new active matrix decoder for surround sound”, In Proc. AES, 19th International Conference on Surround Sound, June 2001) and Circle Surround™ (see U.S. Pat. No. 6,198,827: 5-2-5 matrix system). Another technique of this type is disclosed in U.S. Pat. No. 6,496,584.
An example of the second situation is the improvement of the wideness of the center speaker in a 5.1-system by adding the center signal to the left and right channel. This is done in the music mode of Dolby Pro Logic II™. Another example is stereo-widening, where a small speaker base is used (for example in television systems). Within the Philips™ company, a technique called Incredible Stereo™ has been developed for this purpose.
In the third situation, so-called down-mixing is applied. This down-mixing can be done in a smart way, to maintain the original spatial image as well as possible. An example of such a technique is Incredible Surround Sound™ from the Philips™ company, in which 5.1-surround audio is played back over two loudspeakers.
Two different approaches are known for the redistribution as mentioned in the examples above. First, redistribution may be based on a fixed matrix. Second, redistribution may be controlled by inter-channel characteristics such as, for example, correlation.
A technique like Incredible Stereo™ is an example of the first situation. A disadvantage of this approach is that certain audio signals, like speech signals, panned in the center are negatively affected, i.e. such that the quality of reproduced audio may be insufficient. To prevent such a deterioration of the audio quality, a new technique was developed, based on correlation between channels (see WO 03/049497 A2). This technique assumes that speech panned in the center, has a strong correlation between the left and the right channel.
Dolby Pro Logic II™ redistributes the input signals on the basis of inter-channel characteristics. Dolby Pro Logic II™, however, has two different modes, movie and music. Different redistributions are provided depending on which setting is chosen by the user. These different modes are available because different audio contents have different optimal settings. For example, for movie it is often desired to have speech in the center channel only, but for music it is not preferred to have vocals in the center channel only; here a phantom center source is preferred.
Thus, the discussed prior art concerning redistribution techniques suffers from the disadvantage that different settings are advantageous for different audio contents.
JP-08037700 discloses a sound field correction circuit having a music category discrimination part which specifies the music category of music signals. Based on the music category specified, a mode-setting micro-controller sets a corresponding simulation mode.
US 2003/0210794 A1 discloses a matrix surround decoding system having a microcomputer that determines a type of stereo source, an output of the microcomputer being input to a matrix surround decoder for switching the output mode of the matrix surround decoder to a mode corresponding to the type of stereophonic source thus determined.
According to JP-08037700 and US 2003/0210794 A1, however, the category of an audio content is estimated by a binary-type decision (“Yes” or “No”), i.e. a particular one from among a plurality of audio genres is considered to be present, even in a scenario in which an audio excerpt has elements from different music genres. This may result in a poor reproduction quality of audio data processed according to any of JP-08037700 and US 2003/0210794 A1.